This project has used electrophoretic variants of the X chromosome gene product glucose 6-phosphate dehydrogenase (G6PD) in Mus caroli to examine the expression of X chromosomes in germ cells during oocyte differentiation. We have established that only a single X chromosome is active in germ cells before meiosis begins. Using the appearance of a heteropolymer band of G6PD activity we have determined that the appearance of the two active-X condition is correlated with the onset of meiosis. These findings indicate that primordial germ cells are derived from a pool of cells that have undergone X-chromosome inactivation and that the inactive condition of that X chromosome is reversed during oocyte differentiation. Further characterizations of the timing of reactivation are currently in progress.